Optical dating of a sedimentary sequence in a trenching site on the source fault of the 1999 Chi-Chi earthquake, Taiwan

Abstract

A trenching site was excavated for the purpose of paleoseismic exploration and slip-rate acquisition along the Chelungpu fault, the source fault for the 1999 Chi-Chi earthquake (Mw 7.3) in central Taiwan. In previous studies that used conventional and AMS radiocarbon dating, sample ages clustered in three unevenly spaced groups: <400, 1700–2200, and >30000 BP. Those prior studies also yielded questionable long-term slip rates: the rate for the last 2 ka was six times higher than the average for the past 40 ka. In order to reevaluate the time frame for this trench, optically stimulated luminescence (OSL) dating was applied. The large- and small-aliquot dose distributions were used to investigate the completeness of bleaching on nearby collected modern samples. Results indicate that the 5% minimum ages derived from the small-aliquot dose distributions are preferable for dating young samples, which may include unbleached sediment grains associated with the incompletely bleached sediments. The final OSL dates indicate that the previously published radiocarbon ages include some incorrect ages resulting from contamination and reworking of deposits, and a revised time frame is presented. Cumulative vertical slip independently derived from trench log restoration is 1.8 m in the past 2000 years, which is consistent with the vertical slip rate, i.e., 0.91 m/ka, calculated from the depositional rate in the footwall. This result inversely suggests that the time frame presented in this study is reliable.